Diesel engines are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed up diesel engine is typically in the range of 200° to 400° C. and has a representative composition, by volume, of about 10-17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance nitrogen and water. These NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content and the water content in the hot exhaust stream.
It has been proposed to add gaseous or vaporizable hydrogen-containing substances such as ammonia, urea, or hydrocarbons to the oxygen-rich exhaust gas to provide reactants for the conversion of NOx to nitrogen over a suitable catalyst. The practice is termed selective catalytic reduction of NOx (SCR) and catalysts such as certain base metal-exchanged zeolite catalysts have been tested to accomplish this difficult chemical reduction task. Selective catalytic reduction of NOx by NH3 is one of the most promising technologies known for the control of NOx emissions from lean-burn engine exhausts. Due to the difficulties associated with the storage and transportation of NH3, however, urea has been widely used as a preferred source of NH3 for automotive applications, based on the fact that one molecule of urea can produce two molecules of NH3 through thermal decomposition. The NOx conversion performance of typical catalysts for NH3 assisted SCR is excellent over a wide temperature range above 250° C., but the low-temperature activity below 250° C. remains a major problem.
It is, thus, an object of the present invention to provide an improved method of reducing NOx in such oxygen-rich, nitrogen oxide containing mixtures, especially at temperatures below about 250° C. It is a more specific object of the present invention to provide a method of modifying lean-burn, hydrocarbon fueled engine exhaust with ozone (from plasma treated ambient air) and ammonia (or the equivalent) before the exhaust is brought into contact with a NOx reduction catalyst.